1. Field of the Invention
This invention relates in general to a receiver, and in particular to a receiver for receiving broadcast signals. More particularly, the invention relates to a receiver implementable as a small monolithic device capable of receiving radio frequency broadcast signals in a wide frequency range, less susceptible to noise interference, and features low intermodulation distortion as well as low radiated electromagnetic energy.
2. Technical Background
As high-frequency radio communication technology advances, there has been an increasing demand for high-quality broadband receivers for the reception of RF (radio frequency) broadcasts such as tuners for TV broadcasts. Traditional TV tuners, however, have been unable to cope with the requirements of modem high-quality video and audio standards. On the other hand, new generations of miniature receivers incorporated into personal computer systems that are light and small, such as those receiver devices for use in the notebook computer environments have been developed. However, these devices were constructed using discrete components, their physical sizes were too excessive to be incorporated into the portable computers. As a result, monolithic IC implementations of these devices were developed.
Thus, IC-based receivers have emerged, of which one is depicted in FIG. 1A. FIG. 1A shows the circuit diagram of a conventional IC-based circuit design for a typical TV/VCR receiver. The receiver circuitry included a UHF input filter 20, a VHF input filter 21, two low-noise RF amplifiers 22 and 23, two inter-stage filters 24 and 25, a GaAs tuner IC 26, an UHF resonator 27, a VHF/CATV resonator 29, an IF (intermediate frequency) filter 28, and a prescalar 30. This typical TV/VCR receiver employed two different input filters 20 and 21 for filtering the input signals at the UHF and VHF/CATV categories respectively whose outputs were then fetched to their corresponding differential low-noise amplifiers 22 and 23 for amplification. Adjusted respectively under the corresponding frequency band controlling signals Bu and Bv, amplifiers 22 and 23 amplified their respective UHF and VHF inputs and then sent the amplified signals to the corresponding inter-stage filters 24 and 25 for tracking-filtering. Outputs of filters 24 and 25 were then fed to the tuner IC 26 for subsequent processing.
FIG. 1B details the block diagram of an implementation of the tuner IC 26 employed in the receiver of FIG. 1A. As is illustrated, the tuner IC was consisted essentially of a frequency band switch 261, a UHF oscillator 262, a VHF/CATV oscillator 263, an oscillator buffer 264, a double-balanced mixer 265, and an intermediate frequency amplifier 266. Two categories of signals input to the GaAs tuner IC 26, either VHF or UHF, were selected at the frequency band switch 261 by the proper application of the Bu and Bv controlling signals. Further, the Bu and Bv control signals also controlled the switching of the corresponding resonators external to the IC unit. Frequency locking was achieved by the oscillator buffer outputting to the prescalar 30 (FIG. 1A) in combination with a phase-locked loop controller not shown in the drawing. When the properly-switched output RF signal was input to the mixer 265 together with the local oscillator signal output by the buffer 264, an output is produced by the mixer 265 that was filtered at the IF filter 28 external to the IC device and then fed back to the internal IF amplifier 266. A UN gain control signal Bm was used to control the amplification gain provided by this amplifier.
Though the receiver of FIG. 1A was built around an IC core circuitry, circuit electrical characteristics, however, was less than ideal. In particular, the noise figure of the receiver of FIG. 1A was found to be greater than 8 dB, which required the use of additional circuitry constructed out of discrete circuit elements external to the IC tuner core for improvements. Use of the noise improvement circuitry, however, introduced further disadvantageous effects over the reception characteristics of the receiver at the high UHF bands.
FIG. 2A schematically outlines the use of a conventional IC tuner used in a satellite TV receiver at the frequency range of 950 MHz to 2 GHz. The entire receiver system was built around an IC tuner 40 that constituted the core circuitry of the receiver. In addition to the tuner IC 40, together with a couple of coupling capacitors C.sub.A and C.sub.B, only a resonator 41 was used to construct the receiver system. This was quite a simple receiver circuitry.
As can be seen in the drawing, the IC tuner 40 was made up of an RF amplifier (RFAMP), a mixer (MIX), an oscillator (OSC), and an IF amplifier (IFAMP). In practice, there should be an IF filter connected to the IF output terminal of the IC tuner when operated.
The IC tuner device used in the system of FIG. 2A was one that was fabricated in an enhanced mode gallium arsenide metal semiconductor field-effect transistor (GaAs MESFET) process, which was different from the conventional depletion mode process. Further, the system depicted in FIG. 2A was most characterized by the fact that a multivibrator oscillator such as the one illustrated in the schematic diagram of FIG. 2B was used. When coupled with the proper resonator (resonator 41 in FIG. 2A), negative impedance could be obtained within a wide frequency range, which was a good characteristics for wide frequency range oscillators. In addition, since the output was differential, it could therefore be directly coupled to the double-balanced mixer for input thereto.
Though very simple in construction, however, the receiver system of FIG. 2A suffer a noise figure of greater than 10 dB. What's more, such receiver was only capable of operation in the satellite TV frequency range. General cable or broadcast TV operations were not applicable. Usefulness of such receiver systems was therefore greatly restrained.
FIG. 3 illustrates a conventional integrated tuner system suitable for use in the cable TV environment. This was a system employing a configuration of two frequency conversions. A GaAs frequency-up converter IC circuitry 50 was used to up-convert the frequency of the input RF signal with a frequency of about 50 to 550 MHz. The input RF signal was fed to a low-noise automatic gain-control amplifier 501 governed by the automatic gain control signal AGC. Output of the AGC amplifier 501 was then mixed at the mixer 502 with an oscillator signal generated by the oscillator 503 having output frequency thereof controlled by a tuning signal TUNING. Mixed output of the up converter 50 was then fetched to an IF filter 51. This allowed to pick up the up-converted IF signal of, for example, 700 MHz frequency in the output of the IF filter 51. Output of filter 51 was then mixed at another mixer 52 with another oscillator signal generated by another oscillator 53. Modulation at this node resulted in the picking up of a down-converted signal of, for example, 45 MHz. After processing in the circuitry system of FIG. 3, the system was considered to be generally acceptable for use in the CATV frequency range of about 50 to 550 Mhz only.
More, due to the fact that linear region of transistors was employed to perform the automatic gain control, the controlled range of the gain was limited. This was disadvantageous considering the amplification distortion for large RF signals as well as the uniformity of IF output signal intensity. Further, since two frequency conversions were employed while only one circuitry for implementing the conversion was implemented inside the IC device, more additional discrete circuit components must therefore be used to implement the second of the two conversions. As a result, system simplification gradually became impossible.
As such, conventional receivers were still with many of the disadvantageous problems. First of all, receiver operating frequency range must be increased so that applications thereof can be expanded. Then, to fulfill the requirements of modern high-quality tuners, receiver electronic characteristics such as low noise figure, good amplifier linearity, low intermodulation, as well as low phase noise in the oscillator should be secured. Moreover, in order to be suitable for mobility applications, a receiver must be implemented into monolithic IC devices with least external components. Finally, simple and mature IC fabrication process must be used to fabricate such IC devices in order to reduce costs.